JP2014139671A - Constant magnification lens for vision system camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens assembly for a vision system, allowing for a constant magnification at various focal distances.SOLUTION: A lens assembly 150 resides movably/adjustably along an optical axis OA relative to a sensor 410. In an embodiment, the lens assembly 150 includes a fixed rear lens L2 and a front lens L1 that is moved mechanically to focus an image of an object on the image sensor 410. The lens assembly may alternatively include a liquid lens that is controlled so as to adjust a magnification with respect to a fixed front lens to maintain a constant system magnification. The liquid lens resides between the (fixed) front lens assembly and the image sensor, and may be controlled to focus the image on the image sensor.

Description

RELATED APPLICATION This application claims the benefit of US Provisional Application No. 61 / 745,927, filed December 26, 2012, entitled “Constant Magnification Lens for Vision System Cameras”. The entire disclosure of which is incorporated herein by reference.

The present invention relates to machine vision systems, and more particularly to optical systems used in handheld symbology readers, and methods for using such optical systems.

Vision systems that perform measurements, inspections, object alignment and / or symbology (eg, barcode) decoding are used in a wide variety of applications and industries. These systems acquire images of subjects and objects (typically grayscale or color, and 1D, 2D or 3D) and interconnect these acquired images on-board or remotely. Based on the use of an image sensor that processes using a vision system processor. The processor typically includes processing hardware and non-transitory computer readable program instructions that perform one or more vision system processes and generate a desired output based on the processed image information. This image information is typically provided in an array of image pixels, each having a different color and / or brightness. In the example of a symbology (barcode) reader, a user of an automated process obtains an image of an object that appears to contain one or more barcodes. The image is processed to identify bar code features, and then the bar code is decoded by a decoding process and / or the processor obtains unique alphanumeric data represented by the code. In another type of vision system, the system processor uses various vision system tools (eg edge detector, caliper, blob analysis) to detect edges and other features, thereby enabling recognition of object features Furthermore, based on these characteristics, it is possible to determine the desired information—for example, whether the object is defective or the object is properly aligned.

In some vision systems, the main component is the vision system camera assembly. The camera assembly includes a lens (optical system) and an imager (or “sensor”) that provides an array of image pixel information. The vision system processor described above receives and processes pixel data from the imager / sensor to derive useful vision system information about the imaged scene and / or object. The vision system processor and associated components (eg, data memory, decoder, etc.) may be provided within the camera assembly housing or enclosure, or some or all of these components may be remotely (eg, a PC or other It may be assembled (in a remote self-contained processing system) and linked to a wired or wireless interconnect. Similarly, the camera assembly may include an on-board illuminator that typically surrounds the lens and / or other lighting configurations that provide light to the imaged scene.

In some vision system cameras, it is desirable to provide an autofocus (“autofocus”) capability. Many autofocus arrangements rely on electromechanical actuation to move a fixed lens, while other configurations rely on increasingly different forms of varioptic lens designs, such as so-called liquid lenses.

In the special field of symbolology reading using a sensor-based vision system, a typical reader configuration employs a hand-held unit that is directed to an object that includes a symbol (eg, a 1D or 2D barcode). Such hand-held systems are typically used to track inventory placed on, for example, a warehouse or factory site. In such an environment, the distance between the symbol and the reader may be very variable. This is because a certain object is present at a position relatively close to the user, but other objects are separated (for example, an object placed on a high shelf). The reader may include a conventional autofocus mechanism to allow a clear image to be produced for both near and far objects, but symbols in the far object will have this long focal length. Assuming that the aperture angle of the optical system is too large, it may appear small relative to the entire field of view. Thus, if the symbol size in the entire image is small, it may be difficult to properly decode because the resolution is insufficient compared to the entire field of view grasped by the sensor (that is, the feature / Symbol too small).

It is therefore desirable to provide a vision system camera assembly that can resolve symbols or other features of interest more efficiently at both short and long focal lengths. The camera assembly is preferably adaptable to handheld devices and / or fixed devices.

The present invention overcomes the disadvantages of the prior art by providing a lens assembly that enables constant magnification at both long and short focal lengths for vision systems such as handheld symbology readers. The lens assembly is movably and / or adjustable along the optical axis at a predetermined distance from the image sensor (generally provided perpendicular to the optical axis). In the exemplary embodiment, the lens assembly consists of two lenses L1 and L2 spaced apart from each other along the optical axis. In embodiments, L1 and L2 can be represented by lens groups. The two lenses L1 and L2 define focal lengths f1 and f2, respectively. These lenses satisfy the following relationship. That is, (a) the focal points of L1 and L2 coincide, (b) the aperture stop of the assembly is between the back surface of L1 and the focal point of L1, (c) the magnification of the assembly is constant and equal to f2 / f1, (D) The displacement of the focal position of the assembly is (f1 / f2) ² * (movement along the optical axis of the assembly). In one embodiment, the lens is moved to a selected position relative to the sensor along the optical axis by an actuator (eg, a geared stepping motor or servomotor). The actuator moves in response to a command from a vision processor using a conventional or custom autofocus process and resolves a sharp image of the feature of interest (eg, symbol) on the object at the current focal length. The feature of interest will appear with approximately the same resolution at each of the maximum focal length and minimum focal length and all ranges in between. The sensor's inherent pixel resolution provides sufficient detail to identify and decode symbols over a working distance.

In an exemplary embodiment, a vision system for acquiring an image of an object over a range of focal lengths in a field of view includes an image sensor operably coupled with a vision processor. A constant magnification lens assembly, including a front lens assembly, directed along the optical axis receives light from the scene and sends the light to the image sensor. The front lens assembly has a smaller area (or associated dimension) than the smaller field area (or associated dimension), making it a practical and relatively compact package. The constant magnification lens assembly also includes a rear lens assembly. The front lens assembly and the rear lens assembly are arranged in a fixed spatial relationship between them. By way of example, the front lens assembly and the rear lens assembly are: (a) the focal point of the front lens assembly and the focal point of the rear lens assembly are coincident, and (b) the aperture stop of the constant magnification lens assembly is (C) the magnification of the assembly is constant and equal to the focal length of the rear lens assembly (f2) / focal length of the front lens assembly (f1), and (d) the focal point of the assembly. The displacement of the position is configured and arranged to be (f1 / f2) ² * (movement of the constant magnification lens assembly along the optical axis). This configuration (ie, item (b)) allows the front lens assembly to define an area and / or size that is smaller than the area and / or size of the imaged field of view. The front lens assembly and rear lens assembly can be assembled in a barrel that is moved in a direction toward and away from the image sensor by an actuator that is responsive to the focus process. Alternatively, the constant power lens assembly and its components can be fixed with respect to the camera body / frame, and the sensor assembly (or portion thereof containing the sensor) can be driven by a suitable actuator (along the optical axis) that reacts to the focus process. It can be moved toward and away from the constant magnification lens assembly.

In an exemplary embodiment, a vision system for acquiring an image of an object over a range of focal lengths in a field of view includes an image sensor operably coupled with a vision processor. The constant magnification lens assembly is directed along the optical axis to receive light from the scene and send it to the image sensor. The constant magnification lens assembly includes a liquid lens assembly that extends between the image sensor and the front lens assembly. The lens assembly may be based on at least two iso-density liquids that change interaction based on electrowetting theory, or the lens may include an actuator that changes the shape of the liquid-filled film. Good. The front lens assembly includes one or more fixed lenses, and the (rear) liquid lens assembly includes an interface that uses the input electrical energy to change the magnification m2 of the liquid lens assembly. . The controller selectively adjusts the magnification m2 of the liquid lens assembly to maintain a focal point on the object at a constant system magnification M at each focal length over a range of focal lengths. Illustratively, the front lens assembly and the rear lens assembly are: (a) the focal point of the front lens assembly and the front major surface of the liquid lens assembly coincide, and (b) the magnification is constant and the image from the liquid lens assembly. It is constructed and arranged to be equal to the ratio of the distance (d2) to the sensor and the distance (d1) between the front lens assembly and the liquid lens assembly. The controller is also arranged to iteratively adjust the magnification m2 of the liquid lens assembly until a desired focus is provided at a constant system magnification M.

By way of example, in any embodiment herein, the front lens assembly and the liquid (or rear) lens assembly are: (a) the focal point of the front lens assembly and the front major surface of the rear lens assembly coincide ( d1 = f1), (b) the aperture stop of the constant magnification lens assembly is between the back surface of the front lens assembly and the focal point of the front lens assembly, and (c) the magnification is constant, from the liquid lens assembly to the image sensor Of the distance (d2) and the ratio of the distance (d1) between the front lens assembly and the liquid lens assembly.

In another embodiment, a method for acquiring an image of an object over a range of focal lengths within a field of view is provided along an optical axis and providing an image sensor operatively coupled with a vision processor. Providing a constant magnification lens assembly that receives light from the scene and sends it to the image sensor. The constant magnification lens assembly includes a front lens assembly. The constant magnification lens assembly is iteratively adjusted until the object reaches the desired focus with the image sensor.

The present invention will be described below with reference to the accompanying drawings.

FIG. 3 illustrates a handheld symbology reader and associated data processing and storage system using a constant magnification lens assembly for acquiring images of objects at short and long focal lengths according to an exemplary embodiment.

It is the image of the scene acquired by the image sensor of the reader of FIG. 1, and shows the resolution of the feature of interest at a short focal length.

It is the image of the scene acquired by the image sensor of the reader of FIG. 1, and shows the resolution of the feature of interest at a long focal length.

FIG. 2 is a cross-sectional view of a lens assembly according to an exemplary embodiment for use with the reader of FIG. 1, showing the relative position with respect to the image sensor along the optical axis of the assembly for a relatively short focal length.

FIG. 5 is a cross-sectional view of the lens assembly shown in FIG. 4 where the relative position with respect to the image sensor along the optical axis of the assembly is set for a long focal length.

FIG. 2 is a diagram illustrating a lens focus mechanism and an associated processor that process image data and controls the focus of a lens assembly in the reader of FIG.

2 is a flowchart of an exemplary focus process using a lens assembly in the reader of FIG.

FIG. 3 is a diagram of a lens assembly for a constant magnification vision system including a liquid lens assembly according to an exemplary embodiment.

Thin lens ray of the lens assembly for the constant magnification vision system of FIG. 8 shown in maintaining constant magnification by controlling the magnification of the liquid lens assembly at closer and farther positions, respectively, in an exemplary object. FIG. Thin lens ray of the lens assembly for the constant magnification vision system of FIG. 8 shown in maintaining constant magnification by controlling the magnification of the liquid lens assembly at closer and farther positions, respectively, in an exemplary object. FIG.

FIG. 5 is a lens assembly for a constant magnification vision system including a liquid lens assembly and an aperture stop provided between the liquid lens assembly and a front lens assembly according to another exemplary embodiment.

I. General considerations

FIG. 1 shows a vision system 100 that includes at least one symbolology reader 110 that is handheld as shown or fixed in position relative to the scene being imaged. The reader can define any acceptable housing including the drawn body 112 and grip 114. In this embodiment, the reader has a front window 116 that may include external and / or internal lighting systems (illuminators). The illuminator may include an arrangement and / or combination of illumination elements in any acceptable configuration. In this embodiment, illuminating elements (e.g., high power LEDs) 120, 122 are used as examples to allow for different colors / wavelengths, angles and / or illumination intensities. The illuminator is a conventional aiming LED (not shown) that emits light toward the field of view to properly and adequately image the feature of interest (eg, a bar code or other symbol, also referred to as “ID”). ) May be included. In this embodiment, the reader 110 may include an indicator and an interface panel 130 provided at the rear portion of the main body 112. This panel may include on / off and other switches and lights indicating “good” or “bad” symbol reading (ie, success or failure in symbol reading / decoding). The grip 114 may include one or more trigger buttons 132 that trigger illumination and image capture alongside functions such as toggling the aiming LED. The reader also includes one or more processing circuits, memory, and the like, which are collectively shown as a vision processor 136 (in phantom lines). The processor performs various image processing and image data manipulation / storage functions. Illustratively, the processor 136 receives an image frame rate captured from an image sensor (also shown in phantom lines) in color or grayscale pixels (or other form). The processor looks for an ID feature (or other feature of interest) in the image and then sends the appropriate data to the decoding process to generate a code from the ID feature. These codes are stored and / or sent to the receiver 142 via the communication link 140 (which may be wired or wireless as shown). Receiver 142 is interconnected with data processing and storage system 144 via a network or other link. The system 144 may include a conventional server or PC that executes a suitable application for manipulating and storing code data sent from the reader 110. The architecture of such an application and system 144 will be apparent to those skilled in the art.

The reader also includes a lens assembly 150 (shown in phantom behind window 116) that provides a constant magnification over a range of focal lengths. As an example, the object O <b> 1 having the symbol S <b> 1 is imaged by the reader 110 having the lens 150. The lens 150 is focused on a field of view FOV1 that occupies a place / scale where the symbol S1 is relatively prominent. This scale is sufficient to provide the necessary details for acceptable ID reading. The focal length D1 along the optical axis OA1 is at least within an operating range of about 350 mm and is about 500 mm for illustrative purposes. Similarly, the reader 110 may focus on another object O2 provided at a significantly shorter focal length D2, along the optical axis OA2, approximately 50 mm for illustrative purposes (as shown in phantom). it can. In particular, using the constant magnification lens assembly 150 according to the exemplary embodiment, the scale of the second symbol S2 in the associated field of view FOV2 is approximately the same as the scale of S1 and FOV1. Thus, regardless of the distance within a given distance range, the field of view and the size of the symbols in the field of view remain the same, allowing sufficient detail to obtain a good reading.

2 and 3, the constant magnification principle is further illustrated by respective exemplary images 200 and 300 that simulate the appearance of images acquired at focal lengths of about 50 mm and 500 mm, respectively. The field of view of each image 200, 300 is defined by the outer edge of the rendered image. By using a constant magnification configuration, both images should ideally present approximately the same border to the scene. Similarly, each exemplary symbol region 210, 310 appears to be relatively similar in size relative to the field of view, providing sufficient detail for the processor to find and decode the symbol.

II. Constant magnification with mechanically driven lens

Reference is now made to FIGS. 4 and 5 showing the constant magnification lens assembly 150 in further detail in accordance with an exemplary embodiment. The assembly 150 includes a front lens L1 (focal length f1) and a rear lens L2 (focal length f2) aligned along the optical axis OA with respect to the sensor 410. Sensor 410 typically defines an image plane perpendicular to axis OA. The lenses L1, L2 in this embodiment are positioned on a barrel 420 or other support structure that maintains their relative alignment and spacing at a fixed distance SL relative to each other. Each of the two lenses L1, L2 is specifically designed to establish a series of relationships that guarantee a constant magnification over a range of focal lengths. More specifically, these relationships are as follows.
(A) The focal points of L1 and L2 coincide on the drawn surface (d1 = f1 + f2).
(B) The aperture stop (AS) of the assembly is between the back surface of L1 and the focal point of L1.
(C) The assembly magnification is constant and equal to f2 / f1.
(D) The displacement of the focal position of the assembly is (f1 / f2) ² * (movement along the optical axis of the assembly).

When the aperture stop is arranged at the position defined in (b) above, the size of the front lens can be smaller than the area, length, width, etc. of the object to be imaged and the associated visual field with respect to the diameter, area, etc. Note that it is advantageous. Conversely, if the stop is placed elsewhere (eg, at the focal point f1), it may be necessary to use a front lens that is approximately equal to the size of the desired field of view--for example, in the form of a telecentric lens. Such large lenses are typically disadvantageous when size and placement constraints exist.

It should also be noted that it is clearly envisaged that the drawn lenses L1 and / or L2 may be defined by a lens group having similar or the same refractive power as a single lens element. In various embodiments, such lens groups may provide improved optical aberration correction compared to a single discrete lens element. Therefore, in this specification, the term “lens” should be understood to broadly include an arrangement of a plurality of discrete lenses.

Those skilled in the art of lens design will understand the structure of lens assemblies that satisfy the above relationships (a)-(d). In one embodiment, the value f2 / f1 is about 0.1, although other ratios are clearly envisioned. As an example, both lens groups L1 and L2 define a positive refractive power. For example, lens L1 can be defined with a focal length of about 30-60 millimeters, and lens L2 can be defined with a focal length of about 6-10 millimeters. As shown, placing lenses L1, L2 at a relatively short focal length FD1 (FIG. 3) in assembly 150 is steeper with respect to a given distance DS1 between sensor 410 and rear lens 410. A light beam pattern 430 that diverges is defined. This larger divergence pattern defines a focal point that is aligned with the field of view 440, which is a desirable size to properly image the feature of interest (eg, symbol / ID).

In order to achieve a field of view 540 (FIG. 5) similar in size to the field of view 440 with a longer focal length FD2, the distance DS2 between the rear lens L2 and the sensor 410 is shortened with respect to the distance DS1 described above. Therefore, the light pattern 530 has a smaller divergence. Note that the aperture stop AS is the same at both focal lengths. Thus, by appropriately moving the distance between the rear lens and the sensor, the optical system can focus on a similarly sized field of view with a wide range of focal lengths.

Referring now to FIG. 6, the lens assembly 150 is directed toward and away from the sensor 138 along the optical axis OA (shown attached to an attached imager circuit board that defines the base of the sensor assembly 630). A typical focus mechanism 610 for movement is depicted. Note that the depicted mechanism 610 is exemplary of different possible focus adjustment mechanisms, and other embodiments will be apparent to those skilled in the art. In this exemplary embodiment, mechanism 610 includes an actuator in the form of a motor (eg, servo motor or stepping motor) 620. The motor has an appropriate torque and, if desired, a speed reducer 622 that rotates the pinion gear in either direction of rotation (double arrow 624). The motor rotates a large gear 626 with an inner periphery that meshes with the outer surface of the lens assembly by threads 628 that may have a relatively shallow pitch. Since the gear 626 rotates based on the driving of the motor, the lens assembly is moved in the direction toward or away from the sensor assembly 630 and the accompanying sensor image plane (double arrow 631). In this embodiment, an anti-rotation pin 632 secured to the housing body 112 engages an axially aligned slot 634 within the lens assembly 150 to prevent lens rotation. In this way, the rotation of the gear is fully converted into a linear motion of the lens assembly along the axis OA. In this embodiment, various anti-rotation configurations can be adopted instead. Alternatively, the lens may be rotated to generate a linear transformation for the sensor within the lens assembly 150 using a fixed threaded base (an alternative to the rotating gear 626). In another embodiment, the lens can be fixed and the sensor can be provided on an axially moving base. In general, the system defines relative movement between the lens assembly 150 and the sensor / image plane (138).

The axial position of the lens assembly 150 is determined by the appropriate focus of the image projected on the sensor 138. In one embodiment, the onboard vision processor 136 includes a focus process 640 that can determine when the image is in focus using conventional techniques. For example, contrast reduction at the edge of the acquired image can be used. In the focus process 640, the constant magnification lens assembly 150 (the whole consisting of the fixed front lens L1, rear lens L2 and aperture stop AS) is moved in a number of position steps by the motor 620, and the process 640 is in the acquired image. The best focus position is determined based on the metrics. Other techniques for focusing the lens are also clearly envisioned--for example, determining the distance from the object / captured scene by sweeping the lens position or using a distance sensor and / or rangefinder, The lens can be moved to a predetermined setting. The lens position setting may be based on, for example, an official table or a reference table that determines the lens position setting using these detected distances.

With further reference to FIG. 6, it is clearly envisioned that the constant magnification lens assembly 150 can be secured to the body / frame of the camera. In such an embodiment, sensor assembly 630 (or a portion thereof (eg, sensor 138)) can be moved in a direction toward and away from fixed magnification lens assembly 150 that is fixed in position along optical axis OA. An appropriate sensor actuator (SA) (shown in phantom as box 648) can be used to move sensor assembly 630 as indicated by the associated double arrow. Any acceptable actuation mechanism can be used, including (but not limited to) an electric worm drive, gear drive and / or linear motor. In general, a focus process (eg, stepping, sweeping, distance detection, etc.) can be used as described above to set the appropriate sensor position for a fixed constant magnification lens assembly.

III. Constant magnification focus process

Turning briefly to FIG. 7, an exemplary procedure 700 for adjusting the focus of a constant power lens assembly is shown. This procedure 700 is a simplified example of any acceptable procedure for adjusting focus during setup and / or run time. At step 710, the sensor obtains one or more image frames of the scene and sends these image frames to the vision processor. The processor performs a focus process (640 in FIG. 6), among other processes, to determine whether the image is sufficiently resolved (step 720). If the image is sufficiently resolved (eg, if the symbol can be decoded), then the decision step 730 can determine the focus to the current setting (step 740). If the focus is unacceptable or worse than the focus achieved with the previous adjustment of the lens assembly, the focus process re-adjusts the focus of the lens in a predetermined direction by an adjustment increment (step 750). The selection of a predetermined direction of movement of the lens assembly can be based on predictions in which direction the focus is improved, or based on an analysis of the image showing which direction provides better focus. Good. If the focus is worse than the previous adjustment, the direction of adjustment is reversed accordingly in the next cycle. Adjustments can be made incrementally using an iterative process, but first make rough adjustments based on a determination as to how “blurred” the image is, and then until the final focus is achieved It is envisaged that fine adjustments can be made. Again, this procedure (700) is an example of a wide range of focusing procedures and / or techniques that will be apparent to those skilled in the art.

IV. Constant magnification using liquid lens

An exemplary lens configuration that may be desirable in certain vision system applications is a so-called liquid lens assembly. For example, one form of liquid lens commercially available from Varioptic, France, uses two types of equal density liquids-oil is an insulator and water is a conductor-using electrowetting principles (phenomena). The refractive power of the lens configuration is changed. One example provides a variable range of refractive power of 18 diopters (1 / focal length). Changing the voltage passing through the lens by the surrounding electrical circuit changes the curvature of the liquid-liquid interface, which in turn changes the focal length of the lens. Some significant advantages of using liquid lenses are lens durability (no mechanical moving parts), fast reaction time, relatively good optical quality, low power consumption, and small size It is to be. The use of a liquid lens is desirable because it eliminates the need to touch the lens by hand, simplifying the installation, setup and maintenance of the vision system. Compared to other autofocus mechanisms, the liquid lens has a very fast reaction time. Liquid lenses are ideal for applications where the reading distance changes from object to object (surface) or while moving from reading one object to another.

Recent development of liquid lens technology comes from Opttune AG, Switzerland. This lens uses a movable film covering the liquid container to change the focal length. This lens advantageously provides a larger aperture than a competitive design and operates faster. The focal length / distance of the optical system employing the liquid lens technology can be changed within a predetermined range (for example, 20 diopters) based on the setting of the liquid lens element. To change this setting, a force is applied to the outer periphery of the membrane using an electromagnetic operation according to known techniques.

FIG. 8 shows a generalized lens system configuration 800 for a vision system (see FIG. 1), which includes a liquid lens element L2L oriented along the optical axis OAL. In this exemplary configuration, the entire lens system 800 consists of at least two lenses or two groups of lenses. The first (front) group L1L has a fixed refractive power, and the second (rear) group L2L consists of or includes a liquid lens element having a variable refractive power. Note that in this embodiment, the front lens assembly and the (rear) liquid lens assembly are arranged in a fixed spatial relationship along the optical axis OAL. The liquid lens L2L is driven by a driver or similar processor 810 that supplies current to the lens and adjusts the focus based on the lens focus process 820. The complete range of exemplary parameters for the system is provided below. If exemplary object 830 is placed at a distance Sobj (approximately 20.70 mm in this example) from the optical surface of first lens (group) L1L having refractive power A1, this lens projects an image at the next distance. will do.
S_2 = Sobj / (A1 * Sobj-1) (Formula 1)

According to a non-limiting example, the distance d1 between the optical surfaces of the lens L1L and the lens L2L and the distance d2 between the optical surface of the liquid lens L2L and the image sensor 840 can be fixed. As a result, when the refractive power A_LL of the liquid lens is equal (set) next, the liquid lens L2L projects this intermediate image on the sensor 840. :
A_LL = 1 / (S_2−d1) + 1 / d2 (Formula 2)

The geometric magnification m1 of the first lens (group) is then equal,
m1 = 1 / (A1 * S_obj−1) (Formula 3)
The magnification m2 of the second lens (group) is next equal.
m2 = 1 / (A_LL * (S_2-d1) -1) (Formula 4)
If the liquid lens L2L is now placed at the back focal point of the lens L1L, this expression can also be written as d1 = 1 / A1.

Substituting this into equations (2) and (4), the total magnification M of this system then decreases.
M = m1 * m2 = d2 / d1 (Formula 5)
Where M is the system magnification, m1 is the magnification of the fixed lens group / assembly, m2 is the magnification of the liquid lens group / assembly, d1 is the distance between the respective optical surfaces of lens L1L and lens L2L, and d2 is This is the distance between the optical surface of the liquid lens L2L and the image sensor 840.

This configuration thus produces a constant magnification that is independent of the object distance. FIGS. 9 and 10 show exemplary thin lens ray tracing through the system lens configuration 800 for two object distances S3 (close to the L1L optical surface) and S4 (far from the L1l optical surface), respectively. . The focal lengths F2L and F2L 'of the liquid lens L2L are appropriately adjusted by the driver 810 and the process 820 to project a focused image at a constant magnification M over the expected range of the object distance Sobj.

Therefore, by changing the refractive power (m2) of L2L, the value of m2 that can maintain the value of M at a predetermined level over a changing distance (Sobj) of the object from the system can be set using various techniques. . M2 can be set using the focus process 700 described above. That is, the refractive power of the lens L2L can be adjusted incrementally (iteratively) until a proper focus is achieved for a selected constant value of M for the object 830 at a given distance.

Reference is now made to FIG. 11 illustrating a constant magnification lens system 1100 having a liquid lens assembly L2L according to another embodiment. With reference to FIGS. 8-10, elements similar to the structure and / or function described above are provided with similar reference numerals. As an example, an aperture stop ASL (see also the description relating to the mechanically moved lens above) can be placed between the front lens assembly L1L and the rear liquid lens assembly L2L. The size of the lenses (group / assembly) L1L and L2L is determined by the position of the aperture stop ASL along the optical axis OAL. If the aperture stop ASL is positioned close to the front lens assembly L1L, the size (diameter) of the liquid lens assembly L2L must be increased. Conversely, if the aperture stop ASL is positioned close to the liquid lens assembly L2L, the size (diameter) of the front lens assembly L1L must be increased. Currently, commercially available liquid lenses are generally relatively small in diameter, so the aperture stop is usually placed close to the liquid lens, or the liquid lens assembly (by itself) serves as the aperture stop in the system.
The table below illustrates and describes some generalized parameters for an example of operation of a constant power lens assembly employing a membrane liquid lens.

Focal length of lens 1 f1 = 100 mm
Closest object distance S_near = -200mm
Farthest object distance S_far = -400mm
Middle close image S2 S2_near = 200mm
Middle far * Image S2 S2_far = 133.333mm
Distance between lenses d1 = f1mm
Distance between L2L and sensor d2 = 20mm
L2L focal length for close object f_ll_near = 16.667 mm
L2L refractive power when near (diopter) A_ll_near = 60 diopters Focal length of L2L for a distant object f_ll_far = 12.5 mm
Refractive power of L2L when distant (diopter) A_ll_far = 80 diopters Refractive power range required for L2L (diopters) R = 20 diopters Magnification at close distance m1_near = −1
m2_near = 0.2
M_near = m1 * m2 = −0.2
Magnification at far distance m1_far = −0.333
m2_far = 0.6
M_far = m1 * m2 = −0.2

The vision system having a constant magnification lens described herein advantageously allows for the acquisition of an image of the area of interest at a wide range of focal lengths with sufficient detail and relatively simple adjustment of the lens assembly. It will be clear. This increases the acquisition speed when the distance between objects changes, and the system can encounter objects with different sizes and shapes (and therefore different focal lengths) and handheld and fixed vision systems ( (E.g. used on moving conveyor lines).

The foregoing has been a detailed description of exemplary embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of the invention. The features of each of the various embodiments described above may be combined with the features of the other embodiments described, if applicable, to provide multiple feature combinations in the related new embodiment. Furthermore, while several embodiments of the apparatus and method of the present invention have been described above, what has been described herein is merely illustrative of the application of the principles of the present invention. Various directions and / or orientation terms used herein, such as “vertical”, “horizontal”, “top”, “bottom”, “bottom”, “top”, “side”, “ “Front”, “Rear”, “Left”, “Right” and the like are only used as relative expressions and are absolute relative to a fixed coordinate system such as gravity. It does not represent the direction. Further, as used herein, the terms “process” and / or “processor” should be understood to include a variety of functions and components that are broadly based on electronic hardware and / or software. Furthermore, the processes or processors described can be combined with other processes and / or processors or divided into various sub-processes or sub-processors. Such sub-processes and / or sub-processors can be variously combined according to the embodiments described herein. Similarly, the functions, processes and / or processors described herein may be implemented using electronic hardware, software comprising a non-transitory computer readable medium of program instructions, or a combination of hardware and software. It is clearly assumed that it can be done. Furthermore, although the lens assembly is shown as a unit having at least two spatially fixed lenses and an external actuator, in alternative embodiments, additional lenses and / or other optical elements (eg, filters) It is clearly envisioned that can also be provided. Also, the lenses of the lens assembly can be individually actuated by separate actuation devices (or a gear set coupled with a common motor). In addition, actuation can be accomplished by alternative mechanisms such as linear motors. In addition, the lens assembly may be removable and / or include a self-contained actuator coupled to the camera by a suitable link. For example, in an embodiment using a liquid lens element, placing the fixed lens assembly at the front and the liquid lens assembly at the rear is merely exemplary. If an appropriately sized liquid lens assembly is available, place it at the front of the assembly and place the fixed (or other) lens assembly behind (ie closer to the image sensor and farther from the object being imaged) Position). Accordingly, this description is intended to be taken as an example and is not intended to limit the scope of the invention.

Claims (15)

A vision system for acquiring an image of an object over a range of focal lengths within a field of view,
The vision system includes an image sensor operatively coupled to a vision processor and a constant magnification lens assembly that receives light from a scene and sends it to an image sensor that is directed along an optical axis;
The constant magnification lens assembly includes a front lens assembly;
The front lens assembly has an area smaller than the area of the field of view;
Vision system. The vision system of claim 1, wherein the constant magnification lens assembly further includes a rear lens assembly, wherein the front lens assembly and the rear lens assembly are disposed in a fixed spatial relationship therebetween. The front lens assembly and the rear lens assembly are: (a) the focal point of the front lens assembly and the focal point of the rear lens assembly are coincident, and (b) the aperture stop of the constant magnification lens assembly is the rear surface of the front lens assembly and the front lens (C) the assembly magnification is constant and equal to the focal length of the rear lens assembly (f2) / focal length of the front lens assembly (f1), and (d) of the focal position of the assembly. The vision system of claim 2, wherein the displacement is configured and arranged to be (f1 / f2) ² * (movement of the constant magnification lens assembly along the optical axis). 4. The vision system of claim 3, wherein the front lens assembly and the rear lens assembly are assembled in a barrel that is moved in a direction toward and away from the image sensor by an actuator that is responsive to a focus process. The front lens assembly, the rear lens assembly, and the aperture stop are each fixed with respect to each other, and further include an actuator that moves the image sensor toward and away from the constant magnification lens assembly. Item 4. The vision system according to item 3. And a liquid lens assembly oriented between the front lens assembly and the image sensor, the front lens assembly and the rear lens assembly comprising: (a) a focal point of the front lens assembly and a front of the rear lens assembly; (B) the rear lens assembly has variable refractive power, and (c) the aperture stop of the constant magnification lens assembly is the back surface of the front lens assembly and the focal point of the front lens assembly. And is constructed and arranged to be equal to the ratio of the distance d2 from the rear lens group to the sensor and the distance d1 between the two lens groups with a constant magnification of the assembly. 2. The vision system described in 2. A vision system for acquiring an image of an object over a range of focal lengths within a field of view,
An image sensor operatively coupled to the vision processor;
A constant magnification lens assembly oriented along the optical axis for receiving light from the scene and sending it to the image sensor, the constant magnification lens assembly being oriented between the image sensor and the front lens assembly Including a liquid lens assembly, the front lens assembly having one or more fixed lenses, the liquid lens assembly including an electrical interface that changes a magnification m2 of the liquid lens assembly;
The front lens assembly and the rear lens assembly are arranged in a fixed spatial relationship between them,
The vision system further includes a controller that selectively adjusts the magnification m2 of the liquid lens assembly to maintain the focus on the object at a constant system magnification M at each focal length over a range of focal lengths. system. The front lens assembly and the rear lens assembly are: (a) the focal point of the front lens assembly and the front main surface of the liquid lens assembly coincide, and (b) the constant magnification lens assembly is constant and from the liquid lens assembly to the image sensor. The vision system of claim 7, wherein the vision system is configured and arranged to be equal to the ratio of the distance (d2) to the distance (d1) between the front lens assembly and the liquid lens assembly. The vision system of claim 8, wherein the liquid lens assembly comprises a membrane liquid lens assembly. The vision system of claim 9, wherein the electrical interface comprises an actuator that changes the shape of the membrane. 9. The vision system of claim 8, wherein the liquid lens assembly includes at least two types of equal density liquids, the two liquids changing the interaction between the two liquids in response to an electrical input changed according to electrowetting theory. The vision system of claim 7, wherein the controller is arranged to iteratively adjust the magnification m2 of the liquid lens assembly until a desired focus is provided at a constant system magnification M. The vision system of claim 7, wherein the front lens assembly has an area smaller than the area of the field of view. A method for acquiring an image of an object over a range of focal lengths in a field of view, comprising:
The method includes the steps of providing an image sensor operatively coupled to a vision processor and a constant power lens assembly oriented along an optical axis that receives light from a scene and transmits the light to the image sensor; The constant magnification lens assembly has a front lens assembly;
The method further comprises the step of iteratively adjusting the constant power lens assembly until the object achieves the desired focus with the image sensor. Further comprising installing a liquid lens assembly that is oriented between the front lens assembly and the image sensor, wherein the front lens assembly and the rear lens assembly are: (a) the focal point of the front lens assembly and the rear lens assembly; (B) the rear lens assembly has a variable refractive power, (c) the aperture stop of the constant magnification lens assembly is aligned with the rear surface of the front lens assembly and the front lens assembly. (D) constructed and arranged so that the magnification of the assembly is constant and equal to the ratio of the distance d2 from the rear lens group to the sensor and the distance d1 between the two lens groups. 15. The method of claim 14, wherein